1. Field of the Invention
The present invention relates to an oscillator and a charge pump circuit using the same, and particularly to a current-limited oscillator and a charge pump circuit using the same.
2. Description of the Background Art
Recently, semiconductor circuits with lower power-supply voltages are produced with the advance of process miniaturization techniques. Specifically, power-supply voltages in common logic circuits are around 1.0 V to 1.2 V. However, the thresholds (Vth) of transistors are not remarkably improved despite the reduced power-supply voltages and are still around 0.5 V to 0.7. That is, at present, power-supply voltages are about twice (2Vth) the transistor thresholds (Vth).
Also, storage-type memories, such as DRAMs (Dynamic Random Access Memories), require VPP voltage higher than the power-supply voltage and VBB voltage lower than the GND voltage. Accordingly, semiconductor circuits are provided with charge pump circuits for boosting the power-supply voltage and GND voltage, and the charge pump circuits are provided with oscillators of current-limited type. Conventional oscillators are disclosed in Japanese Patent Application Laid-Open Nos. 7-66693 (1995) and 8-330912 (1996), for example.
When a current-limited oscillator is driven with a relatively high power-supply voltage, the period of the delayed output is not considerably varied by process finish variations and temperature variations and the oscillator offers stable operation. However, when the current-limited oscillator is driven with a low power-supply voltage, the operation tends to be unsteady with the period of the output pulse significantly varied by process finish variations and temperature variations.